The push--push oscillator, in general, uses a resonator which electrically resonates at an oscillating frequency. FIG. 10A through FIG. 10C illustrate this prior art. FIG. 10A and FIG. 10B depict resonators employed in the prior art. The resonator in FIG. 10B is a hairpin resonator which is transformed from the resonator in FIG. 10A. Both the resonators are electrically almost identical. Resonators formed by a transmission line such as a stripline have been widely used. This resonator is disclosed in the document of "Integrated Circuit for Microwave" written by Yoshihiro Konishi and published from "SAMPO" in 1973. FIG. 10C illustrates an entire circuit of the oscillator, which is disclosed in the document of "Push--Push VCO Design with CAD Tools" written by Zvi Nativ and Yair Shur, and published from Microwave Journal in February 1989. Japanese Utility Model Publication No. H06-73910 also teaches this idea. The oscillator in FIG. 10C comprises the following elements.
(a) resonator circuit 102 comprising a transmission line which length being one-half wavelength of a fundamental wave, and both the ends being left open;
(b) oscillator circuit 105 comprising two branch oscillators 103 and 104 employing Colpitts oscillator and the like, the two branch oscillators being electrically identical;
(c) in-phase addition circuit 106 comprising two transmission lines having an equal electrical length and another transmission line; and
(d) output terminal 107.
Two output signals tapped off from circuit 105 include fundamental wave components, and its odd-order as well as even-order harmonics-wave-components. The fundamental wave component and its odd-order harmonic-wave-component are cancelled by circuit 106, whereby they are scarcely supplied to output terminal 107; however, the even-order harmonic-wave-component is added in-phase by circuit 106 and supplied from terminal 107. This oscillator yet has one output terminal as FIG. 10C illustrates and an output signal obtained therefrom is thus nothing but a single phase. Comparing with a regular circuit which obtains even-order-harmonics by multiplying an output signal supplied from a single oscillator, this oscillator needs an in-phase addition circuit which occupies a physically large space. Further, when this oscillator is employed in a PLL frequency synthesizer, the output signal of the oscillator must be supplied to two circuits, i.e. a PLL circuit and an external circuit, although the oscillator has one output terminal, which eventually demands additional circuits.
The conventional oscillator discussed above has the following problems:
(1) The oscillator is not adequate for devices and instruments which need to be smaller in size, because it requires not only large dimensions but also needs a complex structure.
(2) When the oscillator is employed in the PLL frequency synthesizer, the output cannot be fed into a differential amplifier in double phases. As a result, a noise suppression function proper to the differential amplifier cannot be utilized, which entails the needs of improving the noise immunity of the PLL frequency synthesizer.
(3) Since a stripline is used in a transmission line of the resonator circuit, the quality factor (Q) of the resonator is obliged to be suppressed at a certain level. The phase noise characteristics of the oscillator thus remains at unsatisfactory level.
The conventional oscillators and the devices or instruments employing the oscillators have a limit to be downsized, and need improvement in noise characteristics.